The manufacture of a timepiece component comprising at least one part taking the form of a turned part, such as a timepiece pivot arbor, consists in performing chip removal machining operations on a hardenable steel bar, such as bar turning, to define different active surfaces (bearing surface, shoulder, pivots, etc.) and then subjecting the machined timepiece component to heat treatment operations comprising at least one hardening operation to improve the hardness of said component and one or more tempering operations to improve its tenacity. In the case of pivot arbors, the heat treatment operations may be followed by an operation of rolling the pivots of the arbors, which consists in polishing the pivots to the required dimensions. The hardness and roughness of the pivots are further improved during the rolling operation. It will be noted that this rolling operation is very difficult or even impossible to achieve with most materials of low hardness, i.e. less than 600 HV.
The pivot arbors, for example the balance staffs, conventionally used in mechanical timepiece movements are made of steel grades for bar turning which are generally martensitic carbon steels comprising lead and manganese sulphides to improve their machinability. A known steel of this type, named 20AP, is typically used for these applications.
This type of material has the advantage of being easy to machine, in particular of being suitable for bar turning and, after hardening and tempering, has superior mechanical properties which are very advantageous for making timepiece pivot arbors. These steels have, in particular, superior wear resistance and hardness after heat treatment. Typically, the hardness of arbor pivots made of 20AP steel can exceed 700 HV after heat treatment and rolling.
Although this type of material provides satisfactory mechanical properties for the horological applications described above, it has the drawback of being magnetic and capable of interfering with the working of a watch after being subjected to a magnetic field, particularly when the material is used to make a balance staff cooperating with a balance spring made of ferromagnetic material. This phenomenon is well known to those skilled in the art. It will also be noted that these martensitic steels are also sensitive to corrosion.
Attempts have been made to try to overcome these drawbacks with austenitic stainless steels, which have the peculiarity of being non-magnetic, namely paramagnetic or diamagnetic or antiferromagnetic. However, these austenitic steels have a crystallographic structure, which does not allow them to be hardened and to achieve levels of hardness and thus of wear resistance compatible with the requirements necessary for making timepiece pivot arbors. One means of increasing the hardness of these steels is cold working, however this hardening operation cannot achieve hardnesses of more than 500 HV. Consequently, for parts requiring high resistance to wear due to friction and requiring pivots which have little or no risk of deformation, the use of this type of steel remains limited.
Another approach for attempting to overcome these drawbacks consists in depositing hard layers of materials such as diamond-like-carbon (DLC) on the pivot arbors. However, there have been observed significant risks of delamination of the hard layer and thus the formation of debris which can move around inside the timepiece movement and disrupt the operation of the latter, which is unsatisfactory.
A similar approach, described in FR Patent 2015873, proposes to make a balance staff wherein at least the main part is made of certain non-magnetic materials. The pivots may be made of this same material or of steel. It is also possible to arrange for the deposition of an additional layer applied by galvanic or chemical means or by gas phase (for example of Cr, Rh, etc.). This additional layer presents a significant risk of delamination. This document also describes a balance staff fabricated entirely of hardenable bronze. However, no information is provided as to the method for fabricating the pivots. Further, a component made of hardenable bronze has a hardness of less than 450 HV. Such a hardness seems insufficient for performing a rolling treatment to those skilled in the art.
There are also known, from EP Patent Application 2757423, pivot arbors made of an austenitic alloy of cobalt or nickel and having an outer surface hardened to a certain depth. However, such alloys may prove difficult to machine by chip removal. Moreover, they are relatively expensive because of the high cost of nickel and cobalt.